The new system of serologically recognized alloantigens, termed HL-B, recently described from this laboratory will continue to be studied. These antigens are unrelated to HL-A, found on B lymphocytes and monocytes but not T lymphocytes, and are related to lymphocyte defined determinants involved in the mixed lymphocyte culture system. They appear to be similar to the murine la system. Primary emphasis will initially be placed in two areas: (1) characterizing a number of typing reagents with well defined specificities for the different HL-B determinants, by means of appropriate absorptions of pregnancy serums with lymphoid lines and platelets. (2) chemical isolation and structural characterization of the HL-B molecules from B lymphoid lines obtained from selected individuals. These materials should permit the genetics of the HL-B system to begin to be unravelled and to be related to lymphocyte defined determinants through family and population studies. The effect of these reagents on in vitro immune responses will be examined. The interrelationship of HL-B determinants to other B cell surface markers will be explored. This should facilitate the study of the nature of various lymphocyte populations, other than B cells, that manifest the HL-B antigens as differentiation markers.